Nickel-based single crystal alloy and a method of manufacturing the same

ABSTRACT

The present inventions offer a nickel-based single crystal alloy which has a high strength, is easy in conducting the solution heat treatment, hardly gives a harmful phase and is resistant to corrosion at high temperature.  
     A nickel-based single crystal alloy is offered where the composition consists of 7-15 of Co, 0.1-4 of Cr, 1-4 of Mo, 4-7 of W, 5.5-6.5 of Al, 5-7 of Ta, 4-5.5 of Re, 0-0.5 each of Hf and V, and 0-2 each of Ti and Nb in terms of % by weight and residual part substantially consists of Ni wherein said alloy may contain unavoidable impurities

FIELD OF THE INVENTION

[0001] The inventions of the present application relate to anickel-based single crystal alloy and also to a method of manufacturingthe same. More particularly, they relate to a novel and highly strongnickel-based single crystal alloy having excellent strength at hightemperature and anticorrosive property at high temperature which isuseful as a material to be used under high temperature and high stresssuch as turbine blades and turbine vanes for jet engines and gasturbines for industrial purposes.

PRIOR ART

[0002] Nickel-based single crystal alloys have been known asheat-resistant alloys used for high-temperature machinery and tools suchas jet engines and gas turbines for industrial purposes.

[0003] For example, in order to promote the efficiency of gas turbine,it is the most effective method to raise the inlet gas temperature and,for such a purpose, it is essential to improve the high temperaturestrength of the materials used at high temperature and high stress. Fromsuch a viewpoint, there have been more and more improvements in the hightemperature strenght of nickel-based superalloy year by year and, atpresent, with regard to the nickel-based single crystal superalloy,commercial alloys such as ReneN6 and CMSX10 which are called the thirdgeneration single crystal alloys have been developed and used.

[0004] Actually, however, those nickel-based single crystal superalloysof the third generation have problems. The first problem is that theirsolution heat treatment is not easy. In the nickel-based single crystalalloys of the third generation, the amount of chromium is reduced asmuch as possible and, in place of that, rhenium is added in large amountso that a decrease in corrosion resistance by a reduction of the amountof chromium is compensated by rhenium whereby good corrosion resistanceat high temperature is maintained and a high strength at the mediumtemperature of up to 1,000° C. is achieved.

[0005] However, the heat treatment of those nickel-based single crystalalloys of the third generation is becoming to higher temperature andbecoming complicated as well. For example, in the case of CMSX-10, aboutten steps of solution treatments for 45 hours in total at 1,360° C. atthe highest are necessary.

[0006] Accordingly, there has been a practical demand for new technicalmeans whereby strength at high temperature can be improved withoutdeteriorating the corrosion resistance at high temperature and, inaddition, the solution heat treatment is easy.

[0007] In addition to the above problem, there is another problem that,as a material for high temperature for actual, use, a harmful phase isformed within a short period during the use at the temperature of ashigh as not lower than 1,000° C. Then, the third problem is that, due tosaid harmful phase, the strength lowers to the level of same as or evenlower than that of the second generation single crystal superalloys atconditions of high temperature for long time.

[0008] Under such circumstances, the inventions of the presentapplication are to solve the above-mentioned problems in thenickel-based single crystal alloys of the third generation and to offernovel nickel-based single crystal alloy where the solution heattreatment is easy and strength at high temperature and corrosionresistance at high temperature are excellent and also to offer a methodof manufacturing the same.

[0009] Means to Solve the Problems

[0010] In order to solve the above-mentioned problems, the firstinvention of the present application offers a nickel-based singlecrystal alloy, characterized in that, the composition consists of 7-15of Co, 0.1-4 of Cr, 1-4 of Mo, 4-7 of W, 5.5-6.5 of Al, 5-7 of Ta, 4-5.5of Re, 0-0.5 each of Hf and V, and 0-2 each of Ti and Nb in terms of %by weight and residual part substantially consists of Ni where saidalloy may contain unavoidable impurities.

[0011] The present application further offers a nickel-based singlecrystal alloy according to the above-mentioned first invention whereinsaid alloy contains 10-14 of Co, 2-3 of Cr, 1.5-2.5 of Mo, 5-6.5 of W,5.7-6.3 of Al, 5.5-6.5 of Ta, 4.5-5 of Re and 0.01-0.3 of Hf in terms of% by weight.

[0012] The present application furthermore offers a method ofmanufacturing a nickel-based single crystal alloy as mentioned in theabove first or second invention, characterized in that, a solution heattreatment is conducted at the temperature range of 1,310-1,350 and aaging heat treatment is conducted at the temperature range of 850-1,200°C.

[0013] The present application still furthermore offers a method ofmanufacturing the nickel-based single crystal alloy according to theabove-mentioned method wherein the solution heat treatment is conductedwithin ten hours and the aging heat treatment is conducted within thirtyhours; a method of manufacturing the nickel-based single crystal alloywherein each of the solution heat treatment and the aging heat treatmentis conducted in a single step or by means of temperature changes in twoto four steps; and a method of manufacturing the nickel-based singlecrystal alloy wherein a preliminary thermal treatment at from 1,290° C.to lower than 1,310° C. for two hours or shorter is conducted prior tothe solution heat treatment.

EXPLANATION OF THE DRAWINGS

[0014]FIG. 1 is a drawing which shows the heat treatments of TMS-75alloy which is an example of the present inventions.

[0015]FIG. 2 is a drawing which shows the heat treatments of CMSX-10alloy for comparison.

[0016]FIG. 3 is a drawing which shows the result of a creep testarranged in accordance with Larson-Miller's parameter.

[0017]FIG. 4 is a drawing which shows the result of a creep tests at thetemperature of 1040° C. and 1100° C. (stress: 137 MPa)

EMBODIMENTS OF THE INVENTIONS

[0018] The inventions of the present application are characterized asthe nickel-based single crystal alloy having the above-mentionedspecific composition and noticeable functions such as that the solutiontreatment is far easier as compared with the conventional methods andthe time required for the heat treatment can be made shorter, thatharmful phase is hardly formed even when used at high temperature forlong time, that the microstructure is stable and the strength at hightemperature is high, and that corrosion resistance at high temperatureis good can be achieved.

[0019] The nickel-based single crystal alloy of the present inventionshaving such characteristic features can be manufactured in such a mannerthat starting materials in a predetermined composition are dissolved,made into single crystals by, for example, means of a directionalsolidification as in the conventional methods and subjected to asolution heat treatment and aging heat treatments. The treatments atthat time may be conducted under far more simple conditions as comparedwith the conventional methods.

[0020] As mentioned already, the solution treatment may be conductedwithin a temperature range of 1,310-1,350° C. and the aging treatmentmay be conducted within a temperature range of 850-1,200° C. as asuitable embodiment. In those heat treatments, changes in the settemperature may be sufficient in a single step or in two to four steps.The solution heat treatment may be conducted within ten hours or ratherwithin about six hours while the aging treatment may be conducted withinthirty hours or rather within about twenty-six hours. The term “about”used here stands for ±two hours.

[0021] Prior to the solution heat treatment, a preliminary heattreatment at from 1,290° C. to lower than 1,310° C. for within two hoursmay be conducted so that switching to the solution heat treatment cantake place smoothly.

[0022] As such, the nickel-based single crystal alloy of the presentinventions can be achieved by the heat treatments which are far moresimple as compared with those in the conventional methods . As a resultof the heat treatments, all γ′-phases solutioned containing small amountof eutectic γ′-phase are solutioned and then fine coherent γ′precipitation takes place forming cubes whereby no harmful phase wasformed.

[0023] The solution heat treatment is to solution the γ′-phase and theeutectic γ′-phase existing in single crystal into a γ-phase which is amatrix phase. Then, as a result of aging heat treatment, amicrostructure wherein fine γ′ particles are homogeneously precipitatedand dispersed is obtained.

[0024] Limitation of the composition (% by weight) of the alloy is dueto the following reasons.

[0025] Co is 7-15%. When the amount is more than 15%, amount of the γ′phase becomes smaller and the strength is lowered. When it is less than7%, the temperature range for giving solid solution becomes narrower.Therefore, it is necessary that the amount of Co is 7-15% and,preferably, 10-14%.

[0026] When the amount of Cr is more than 4%, a TCP phase which is aharmful phase is formed while, when it is less than 0.1%, corrosionresistance at high temperature lowers. Therefore, it is necessary thatthe amount of Cr is 0.1-4% and, preferably, 2-3%.

[0027] When the amount of Mo is more than 4%, a TCP phase is formedwhile, when it is less than 1%, a raft effect obtained by making themisfit of γ and γ′ minus is insufficient. Therefore, it is necessarythat the amount of Mo is 1-4% and, preferably, 1.5-2.5%.

[0028] When the amount of W is more than 7%, a TCP phase is formedwhile, when it is less than 4%, strength of the solid solution isincomplete whereby creep strength lowers. Therefore, it is necessarythat the amount of W is 4-7% and, preferably, 5-6.5%.

[0029] When the amount of Al is more than 6.5%, amount of the eutecticγ′ phase becomes much and the solution heat treatment is difficultwhile, when it is less than 5.5%, amount of the precipitated γ′ phasebecomes small whereby the strength lowers. Therefore, it is necessarythat the amount of Al is 5.5-6.5% and, preferably, 5.7-6.3%.

[0030] When the amount of Ta is more than 7%, amount of the eutectic γ′phase becomes much and the solution heat treatment is difficult while,when it is less than 5%, strength of solid solution of the γ′ phase isinsufficient whereby the strength lowers. Therefore, it is necessarythat the amount of Ta is 5-7% and, preferably, 5.5-6.5%.

[0031] When the amount of Re is more than 5.5%, a TCP phase is formedand, in addition, temperature range for giving solid solution becomesnarrower while, when it is less than 4%, solid solution strength of theγ phase is insufficient whereby the strength lowers. Therefore, it isnecessary that the amount of Re is 4-5.5% and, preferably, 4.5-5%.

[0032] Besides the above, Hf, V, Ti and Nb which are the minore elementsmay be added, either solely or jointly, thereto for improving thestrength.

[0033] With respect to Hf and V, each of their amounts is 0-0.5% and,with respect to Ti and Nb, each of their amounts is 0-2%. Although eachof those elements is not essential, it is preferred that at least one ortwo of them is/are added within the above-mentioned range. Morepreferably, it is appropriate that at least Hf is added in an amount of0.01-0.3% by weight.

[0034] Further, C and B may be added to prepare a nickel-baseddirectionally solidified (DS) alloy or an conventionally casting (CC)alloy. In that case, addition of 0.01-0.2% by weight of C (carbon) and0.001-0.05% by weight of B (boron) is suitably considered forstrengthening the crystal grain boundary.

[0035] Residual part is substantially Ni and may contain unavoidableimpurities.

[0036] Embodiments of the inventions of the present application will befurther illustrated by way of the following examples.

EXAMPLES

[0037] A composition consisting of 12.0 of Co, 3.0 of Cr, 2.0 of Mo, 6.0of W, 6.0 of Al, 6.0 of Ta, 5.0 of Re and 0.1 of Hf in terms of % byweight where the residual part was substantially consisted of Ni wasprepared and subjected to a preliminary melting in a high-frequencyvacuum melting furnace to prepare a molting stock. This was poured intoa single crystal mold on a water-cooled copper plate heated by agraphite heater and then the mold was pulled down under the heater atthe rate of 200 mm/hr so that the melted metal in the mold with selectorwas subjected to a directional solidification to the upper directionfrom the water-cooled copper plate making into single crystal to give analloy sample (TMS75).

[0038] A composition consisting of 3.0 of Co, 2.0 of Cr, 0.4 of Mo, 5.0of W, 5.7 of Al, 8.0 of Ta, 6.0 of Re, 0.03 of Hf, 0.1 of Nb and 0.2 ofTi in terms of % by weight where the residual part was substantially Niwas made into another alloy sample (CMSX10) which was a conventionalalloy and used as a comparative sample.

[0039] The above samples were subjected to the evaluation for thefollowing items. They were: {circle over (1)} temperature range wherethe solution heat treatment was possible was determined; {circle over(2)} a creep test; {circle over (3)} the sample was completely dipped ina melted salt of 25% NaCl+75% Na₂SO₄ at 900° C. for 20 hours and thecorrosion resistance at high temperature was evaluated; and {circle over(4)} the microstructure after heating at 1,100° C. for long time (1,000hours) was evaluated.

[0040] As a result, it was ascertained that TMS75 had a solution heattreatment range of about 40° C., i.e. from 1,310 to 1,350° C. Because ofthat, as shown in FIG. 1, the solution treatment of the TMS75 alloy wasconducted by heating at 1,300° C. for one hour, for the sake ofprecaution followed, by keeping at 1,320° C. for five hours and coolingby air. After that, a two-step aging treatment was conducted. In thefirst aging step, the alloy was kept at 1,150° C. for four hoursfollowed by cooling by air and, in the second aging step, it was kept at870° C. for two hours followed by cooling by air. With regard to CMSX10,a solution treatment and an aging treatment were conducted according toa conventional method as shown in FIG. 2.

[0041] Result of the creep test after the heat treatments was as shownin FIG. 3 arranged in accordance with Larson-Miller's parameter(right-hand side of the abscissa is higher temperature side; ordinateshows testing stress) and, at the side of low temperature and highstress, the alloy which was developed by the prevent inventors was inthe same level as the comparative alloy CMSX10 while, at the side ofhigh temperature and low stress, it showed better creep characteristicsthan CMSX10.

[0042]FIG. 4 shows results of the creep test at the temperature of 1040°C. and 1100° C. The stress is 137 MPa. It was found that TMS-75 of thepresent invention showed superior creep characteristics.

[0043] Corrosion resistance at high temperature was evaluated bycompletely dipping the sample in a fused salt of 25% NaCl+75% Na₂SO₄ at900° C. for 20 hours. Size of the sample was made 6 mm in diameter and4.5 mm in length. It has been said that, in general, the less the amountof Cr added, the better the corrosion resistance at high temperature.However, the result was that, as compared with the MarM247 cast alloy(containing 8.3% of chromium) which was tested at the same time, wholeamount (not shorter than 3 mm calculated as the metal loss by thecorrosion) of the MarM247 cast alloy was corroded before the term of 20hours already while TMS75 alloy showed excellent corrosion resistance(0.01 mm) at high temperature.

[0044] In the evaluation of the microstructure after heating at 1,100°C. for as long as 1,000 hours, a harmful phase (which was believed to besame as that isolated in CMSX10—a single crystal superalloy of the thirdgeneration) was formed even in the case of TMS75 alloy. However, theratio of the area including the part affected by this harmful phase wasnot less than 20% when heated at 1,100° C. for 1,000 hours in the caseof CMSX10 while, in the case of TMS75 alloy, it was only 4% whereby thelatter alloy had a stable microstructure where from a harmful phase washardly formed.

[0045] Merit of the Invention

[0046] As fully illustrated hereinabove, the inventions of the presentapplication offer a nickel-based single crystal alloy which has a highstrength, is easy in conducting the solution heat treatment, hardlygives a harmful phase and is resistant to corrosion at high temperature.

[0047] It is to be particularly noted that the solution treatment isvery simple in the alloy of the present inventions and accordingly thatthe time required for the heat treatment can be made shorter. Moreover,even when it is used at high temperature for long time, a harmful phaseis hardly formed, the microstructure is stable and the strength at hightemperature is high.

[0048] When the nickel-based single crystal alloy having thoseadvantages in accordance with the present inventions is utilized as amaterial for turbine blade and turbine vane, it is now possible toimprove the thermal efficiency and output of the machinery and toolsused at high temperature such as gas turbine for electricity generationand jet engine.

What is claimed is:
 1. A nickel-based single crystal alloy,characterized in that, the composition consists of 7-15 of Co, 0.1-4 ofCr, 1-4 of Mo, 4-7 of W, 5.5-6.5 of Al, 5-7 of Ta, 4-5.5 of Re, 0-0.5each of Rf and V, and 0-2 each of Ti and Nb in terms of % by weight andresidual part substantially consists of Ni where said alloy may containunavoidable impurities.
 2. A nickel-based single crystal alloy accordingclaim 1 wherein said alloy contains 10-14 of Co, 2-3 of Cr, 1.5-2.5 ofMo, 5-6.5 of W, 5.7-6.3 of Al, 5.5-6.5 of Ta, 4.5-5 of Re and 0.01-0.3of Hf in terms of % by weight.
 3. A method of manufacturing anickel-based single crystal alloy mentioned in claim 1 or 2 ,characterized in that, a solution heat treatment is conducted at thetemperature range of 1,310-1,350° C. and a aging heat treatment isconducted at the temperature range of 850-1,200° C.
 4. A method ofmanufacturing a nickel-based single crystal alloy according to claim 3wherein the solution heat treatment is conducted within ten hours andthe aging heat treatment is conducted within thirty hours.
 5. A methodof manufacturing the nickel-based single crystal alloy according toclaim 3 or 4 wherein each of the solution heat treatment and the agingheat treatment is conducted in a single step or by means of temperaturechanges in two to four steps.
 6. A method of manufacturing thenickel-based single crystal alloy according to any of claims 3 to 5wherein a preliminary heat treatment at from 1,290° C. to lower than1,310° C. for two hours or shorter is conducted prior to the solutionheat treatment.